Let me play the devil's advocate a bit. Sorry if it looks too informal.
You have not lost your English. Instead, you have acquired a new language (well, a dialect).
And its name is Simple English.
Living in your home country, you naturally communicate using your local dialect. Be it American English, British English, etc. Note that another, non-local dialect still may cause a bit of problem, e.g. for different idiomatic constructs.
Living in a foreign country, you are surrounded with people who use Simple English, obviously because English is L2 for them. This makes you to change your habits of using your own language, otherwise they would misunderstand you too often. Ability of «speaking rather slowly, using simple words» is essentially what is meant here.
So, you have really acquired another language.
And you have not lost your L1. Simple proof is that you still comprehend it when listening or reading.
The real challenge is obtaining an ability to quickly switch between the languages (dialects), and this is where you should probably point your efforts to.